In one type of known exit sign, right or left directional arrows are partially formed in the sign stencil, but maintained within the stencil voids or perforations attached by tabs (arrow knockouts). At the time of sign installation, the workman removes one of the arrow knockouts, for example, the left one for a left indicator. However, if the workman removes the wrong arrow knockout, it cannot be replaced and another sign must be used. The "wrong" arrow knockout sign must either be used in another location (if there is one) or discarded as useless.
There are some known exit signs in which mistakes at installation are correctable without the sign becoming useless. Thus, U.S. Pat. No. 3,402,494 discloses an exit sign in which the sign stencil has a void in the shape of a double headed arrow and a cover with an arrow tail imprinted thereon for mounting on and covering one of the arrow heads so as to give the impression of a composite arrow with head and tail pointing in the same direction. However, the mounting hardware acts in an interference fashion to score or broach during insertion such that removal is difficult and reuse is unlikely. In addition, the installed cover projects from the stencil surface and is therefore visible and has a third appearance.
Other known exit signs, exemplified by U.S. Pat. Nos. 3,478,455 and 3,665,626, have an exit sign stencil with one or more rectangular voids and an arrow mask stencil slidably arranged behind the rectangular void so as to present in register with the rectangular void a directional arrow in one position and a blank cover in another position. These slidable mask members can be easily tampered with on the job and are subject to being jarred loose or out of position so as to mistakenly present the wrong image.